Through our 2-year investigations, the following were revealed :(1) Mechanisms underlying IL-12 production : IL-12 production is mediated by antigen-presenting cells/macrophages through stimulation with either CD40 ligand expressed on activated T cells or lipopolysaccharide (LPS). We have demonstrated that IL-12 production via these two pathways is regulated by cellular and soluble components involved in humoral immunity ; B cells and Th2 type cytokines (IL-10, IL-6 and IL-4).(2) Establishment of an IL-12-responsive T cell clone (2D6) : We succeeded in establishing a T cell clone which can respond to a small amount of rIL-12 and be maintained in vitro cultures with rIL-12 alone. Establishing this clone permitted us not only to prepare a high-sensitive bioassay system capable of quantitating the amounts of IL-12 but also to investigate the following study.(3) Signaling through IL-12-receptor (IL-12R). Stimulation of T cells with IL-12 rsults in the phosphorylation of two Janus kinases (TYK2 and JAK2) and two STAT proteins (STAT4 and STAT3). 2D6 cells could be maintained with either IL-12 or IL-2 (designated 2D6^<IL-12>, or 2D6^<IL-2> respectively). We found that stimulation of 2D6 with IL-12 activates STAT5 in addition to the above signaling molecules. Using these two 2D6 sublines, we observed that IL-12 stimulation induces proliferation in both sublines, but IFN-gamma production only in 2D6^<IL-12>. Our study also showed that TYK2 and STAT4/STAT3 are required for IL-12R signaling to IFN-gamma production whereas JAK2 and STAT5 are responsible for including the proliferation of 2D6 cells.